Document additional parameters for read_graph and write_graph

[szhorvat]

Additional parameters available for specific formats for read_graph and write_graph should be documented.

Is this something you could have a look at @maelle ?

[maelle]

I first thought looking at the linked C docs might help, but for Pajek I see more arguments in the C docs than what is supported in the R code

https://igraph.org/c/html/latest/igraph-Foreign.html#igraph_read_graph_dimacs_flow

read.graph.pajek <- function(file, ...) {
  if (length(list(...)) > 0) {
    stop("Unknown arguments to read_graph (Pajek format)")
  }

So I'll concentrate on what's in the R code.